Higher levels of integration in VLSI circuits have led to exposure patterns of increasingly small size. This has created a need for exposure light sources of shorter wavelength in the lithography systems or steppers used to write circuit patterns on semiconductor wafers. As a result, the mainstream light source in exposure tools has changed from the prior art KrF excimer laser (wavelength 248 nm) to ArF excimer laser (wavelength 193 nm). Recently the ArF immersion lithography is in practical application.
In the photolithography technology of the ArF immersion generation, it becomes the key to control the birefringence of photomask substrates. The photolithography technology adopts a system of transferring a photomask pattern to a resist film on a wafer by using ArF excimer laser as the light source, directing polarized illumination through a photomask substrate, and thus exposing the resist film patternwise. For transferring a pattern of finer feature size, it becomes important to improve the contrast. The contrast improvement is affected by such factors as flatness and birefringence of the photomask substrate. Birefringence manifests by residual strains or the like in quartz glass. If birefringence is significant, the polarization of light in the ArF immersion lithography tool is disturbed, leading to a decline of exposure performance.
For this reason, active research works are made how to control the birefringence of synthetic quartz glass substrates for photolithography. For example, Patent Document 1 discloses a mask blank for use in a semiconductor device fabrication technique employing an exposure light wavelength of 200 nm or shorter, comprising a synthetic quartz glass substrate and a light-shielding film laminated on the surface thereof, the mask blank having a birefringence value of 1 nm or less per substrate thickness at wavelength 193 nm. Patent Document 2 describes a process for producing a synthetic quartz glass having a birefringence value of 0.3 nm/cm or lower on the average at wavelength 633 nm. Patent Document 3 describes to heat treat synthetic quartz glass such that the major surface of a synthetic quartz glass block may have a maximum birefringence value of up to 2 nm/cm over its entirety.
Also, glass substrates used in the nanoimprint lithography (NIL) are required to have a high accuracy of shape or topography. The NIL is a technique of imprinting a nano-structured pattern to a resin for pattern transfer. The resolution of the pattern to be transferred depends on the resolution of nano-structures on the mold. Then the substrate on which a fine feature pattern is formed is required to have a high accuracy of shape. As mentioned above, birefringence manifests by residual strains or the like in quartz glass. If birefringence is significant, the substrate surface undergoes substantial changes of flatness and parallelism by the impact of residual stress, before and after processing synthetic quartz glass to a shape for NIL substrate. Such changes may lead to a focal shift during exposure and a pattern shift during transfer. To solve this problem, Patent Document 4 proposes a synthetic quartz glass substrate for microelectronic use having a maximum birefringence value of up to 3 nm/cm in its entirety.
Besides, the synthetic quartz glass members to be assembled in exposure tools and various other equipment used in the processes of manufacturing microelectronic and display components are also required to have a high purity and accuracy.